(1) Field of the Invention
The present invention relates to primary flight controls for main and/or tail rotors of helicopters with the features of the preamble of claim 1.
(2) Description of Related Art
The known primary flight controls allow transmission of a pilot's commands to the control surfaces of an aircraft to initiate and execute maneuvers of the aircraft during operation. At first these primary flight controls were entirely mechanical transmitting the commands of the pilot from the control stick by means of push and pull rods or cables to the control surfaces of the aircraft. Second generation primary flight controls are having power amplification means to help the pilot transmitting mechanically his commands to the control surfaces of the aircraft. Third generation primary flight controls are having electric systems at the place of the push and pull rods or cables and said electric systems are conceived to transmit electrically the pilot's commands to the control surfaces of the aircraft. Flight control computers in these electric systems define and control the commands to the control surfaces. Said electric systems of the third generation allow a considerable reduction of the operational weight of an aircraft, particularly for civil aircrafts, and an important increase in inherent system safety as said electric systems allow multiple distribution lines contributing to fail-safe redundancy of the primary flight controls. The flight control computers are able to add numeric support to the pilot's commands and subsequently contribute to improved primary flight controls of aircrafts and to improved comfort for the pilot.
As far as helicopters are concerned there are special requirements to the primary flight controls, namely the particularly exigent requirement that not more than one control surface is in control for each degree of freedom of the helicopter and said control of the control surface has to be refitted immediately in case of any malfunction of the primary flight controls to keep control of the helicopter. Additionally—compared to airplanes—helicopters are much more sensitive in response to any input to their primary flight controls and said inputs are more frequent. As a matter of fact with regard to primary flight controls there are three major differences between airplanes and helicopters:                airplanes generally have several control surfaces per degree of freedom and additionally there are several actuators available for each control surface. Thus if the control of one control surface shows any malfunction another control surface may take over instead of the deficient control surface. Contrary to airplanes helicopters have not more than one control surface per degree of freedom of the helicopter and said control of the control surface has to assure the safety of the helicopter without any backup.        the elements of the primary flight controls have to carry much higher loads than corresponding elements of an airplane as said loads may be distributed to several corresponding elements for one control surface of an airplane, and        airplanes generally have a certain inherent stability allowing the primary flight controls some spare time for reconfiguration in case of any malfunction. Contrary to airplanes helicopters are inherently instable requiring constant control to avoid destabilisation.        
Taking into account size and complexity of mechanical primary flight controls there are considerable advantages as to weight coming with electrical primary flight controls in airplanes. Such weight advantages are not paramount for helicopters due to their concept and to the design of their fuselage. On top some flight attitudes ask high processing power as not only the pilot's commands have to be taken into consideration, but also helicopter conditions, such as angular speeds, in order to improve piloting and most of all stability of the helicopter. Such high processing power can only be provided by digital computers, said digital computers being susceptible to malfunctions originating from many causes, e.g. hardware damage, logical errors or susceptibility with regard to radiation.
All of the above mentioned difficulties hamper the introduction of electric primary flight controls in helicopters. Preferred primary flight controls for main and/or tail rotors in helicopters of the state of the art comprise:                at least one control element such as a control stick to be handled by the pilot, said control element interacting with push and pull rods and/or cables of the mechanical primary flight controls for actuation of the control surface correspondingly,        at least one gear element integrated into the mechanical primary flight controls and responsive to the pilot's input and the automatic pilot without any feedback to the pilot,        resetting means, acting via a soft link, like a spring, being responsive to the pilot's input and the automatic pilot, and        a computer determining the sequence of commands transferred to the gear element and the resetting means.        
The gear element has to be able for rapid action for efficient stability. In case of any malfunction the gear element may receive a false command of the mechanical primary flight controls and the subsequent transmission to the control surface might put at risk the helicopter. To obviate said risk there are surveillance circuits, alarms, etc. The gear element is generally to slow in its operational frequency and there may be uncomfortable feedback from the actuation of the gear element to the control stick. Superposed to the actuations of the pilot are the actuations of said resetting means destined to stabilize the helicopter. The frequency of the actuations of said resetting means is for security reasons even much slower than the frequency of the actuations of said gear element. Said mechanical primary flight controls give high reliability as to their output but little comfort for the pilot compared to any electric primary flight controls. Mechanical primary flight controls are increasingly unable to correspond to the requests of modern helicopters with high profiled operations.
The document FR 2771071 A1 discloses a device using a computer to interpret control force applied by the pilot and applying relevant control to an actuator. The control for operating the control within a helicopter includes a control handle or joy stick which moves a steering linkage. This is linked to a device for operating the control. A jack is built into the steering linkage and a computer determines the control instructions which are transmitted to the jack. A sensor measures the values of a parameter representative of the force exerted by the pilot on the control handle. The computer then uses these measured values to determine the control instructions. A further mechanism is provided to apply a centering force to the control handle. Said control is entirely mechanic and uses electric surveillance to verify specific parameters.
The document FR 2761659 A1 discloses a flight controller having multiple command generation systems and multiple servo-systems, which receive flight data and flight control instructions. Each of the command generation systems generate a set of commands to direct operation of the flight control surfaces, and one at least of them performs a self-monitoring operation and generates corresponding monitoring signals. Each of the servo-systems has a selector which receives the data from the control surfaces and also the control commands. The selector operates under a set priority system, and takes account of the monitoring signal, automatically blocking any system that indicates a fault. Said controller is electric and requires a lot of redundancy to avoid malfunction with the consequence of extremely complex electric architecture to achieve a maximum of safety. Considerable fire protection means were compulsory for all the components of the controller situated in any fire hazard area with the consequence of bulky components. Costs of development for said electric flight controller were extremely high.